Welcome to AA, lilcam!
Good question!
Advocates of salt say that it lowers stress by making it slightly easier for the
FW fish to osmoregulate. In addition, the chloride ions present in dissolved sodium chloride, reduce the poisonous effects of nitrite. Taken at face value, this seems like a no-brainer, good thing to do.
But, salt is not tolerated well by scaleless fish like corys, by most
FW inverts, and by some
FW plants. Critics point out that its addition adds another variable to the tank that must be monitored and taken into account. Even with partial water changes, the salt levels may climb due to ongoing evaporation. This would happen to a larger degree in small tanks.
I used to add salt when I first set up my tank, but I stopped because the consensus opinion here was against it. I've found that ost people here subscribe to the
K.I.S.S. philosophy regarding water additives. I don't think the fish miss salt and I don't think it would have prevented any of the problems I have run across since I stopped using it. I stopped using carbon a long time ago for the same reasons: it never produced a noticeable benefit and its absence never resulted in any noticeable problems.